Skin - The Jack of All Trades - Class 9 Biology PDF

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(b) The granular layer is a very thin middle layer. List the four ways in which the skin serves as a protective layer for our body. consisting of two or three sub-layers of flattened. 2. Cross out the items in the following list which cells. Gradually, it gives way to the outermost are not the functions of the human skin : - cornified layer. Sensation, respiration, storage of glycogen, (c) The malpighian layer (stratum malpighi, also excretion, digestion, temperature regulation, called germinative layer) is the innermost synthesis of vitamin D, secretion of hormone region of the epidermis. Its cells can actively divide to produce new cells which press and 13.3 STRUCTURE OF THE SKIN (Fig. 13.1 & 13.2) shift outward to replace the worn-out cells of the The principal components of the human outermost cornified layer. (mamdim) skin are as follows. Colouration of the skin (e.g. the complexion of the face) is due to a pigment melanin contained in the A. Skin Epidermis — cornified, granular and Proper —— germinative layers. cells of the malpighian layer. The different quantities Dermis — elastic fibres, blood of this pigment in different human races lead to vessels, nerves, etc. form very light-brown to dark colouration of the Hair skin. African negroes have genetically dark coloured B. Derivatives Nails skin, the Europeans light coloured (whitish) and of the skin —— | Mammary glands most Indians have an intermediate coloured Sweat glands Sebaceous glands (wheatish) skin. In fair-coloured people also, on continued exposure to sunlight, the face or other 13.4 THE SKIN PROPER parts of the body may get tanned, but revert to Microscopically, the skin proper is composed original condition when the exposure is cut down. of two layers — the outer epidermis and the inner dermis. FREE NERVE HAIH SHAF T ENDING. 13.4.1 Epidermis GRANULAR Laver |® The epidermis is the outer thinner part of the skin. It is formed of stratified epithelium piled up 5, [ STRATUM & MALPIGH | ©) layer after layer. At places, the epidermis becomes ~—\/— EPIDERMIS thick and hard as on the palms, soles and spécially on the heels. It is devoid of blood vessels at all places. The epidermis shows three regions (or sublayers) depthwise : (a) outermost cornified layer, ®@ g3 I3 g& (b) middle granular layer and (c) inner malpighian layer. (a) The cornified layer (stratum corneum) is the DERMIS outermost layer consisting of several piled up { FOLLICE layers of flattened dead cells. These cells are FAT CELLS made of a fibrous structural protein called - keratin (also found in nails, hairs, horns, hooves < ilz | and silk). Its cells are continually worn away or shed and are replaced from beneath by those SWEAT GLAND ERECTOR NERVE MUSCLE OF HAIR arising from the deeper malpighian layer. 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(i) Plate — It is the hard and outer part of the The hair follicle is a structure enclosing the nail. It is made up of dead, keratinized cells, hair root. It is composed of an_epithelial and a (ii) Bed (root) — It lies below the nail plate. connective tissue sheath. The hair bulb and the hair’ (iii) Matrix — It lies just below the skin surface at follicle together ate responsible for the growth and the base of the nail. It is usually visible as elongation of the hair. The growth of hair occurs by whitish half-moon at the base of nail, it addition of célls at the base, which soon die.., produces new cells which on maturation push Hair follicles contain pigment cells that produce out the older cells towards the tip of the nail melanin. The colour of hair is due to varying and causes growth of the nail. quantities of melanin. When melanin production decreases, hair become grey, silvery or even 13.5.3 Sebaceous Glands completely white. These branched glands usually open into a hair Everyone has sometimes experienced “goose follicle, though sometimes they even open directly to flesh” during winter or during some emotion. In the outside. They give out an oily secretion (called this, the hair is lifted called piloérection, ( pilo : sebum) which makes the hair ang the outer surface hair) and the surface of the skin presents a of the skin oily and waterproof to keep the epidermis somewhat contracted and wrinkled appearance. This supple and to prevent loss of moisture by evaporation. is caused by the erector (also called arrector) In cold and dry weather, the skin may become rough muscle of the hair which runs obliquely between the and leave a powdery surface when scratched; this is hair follicle and the outer part of the dermis. _The due to a reduced secretion of oil from the sebaceous contraction of this muscle at one end pulls “the hair glands. In hot and humid weather, the skin becomes to a somewhat "vertical position, and at the other extra oily due to an increased secretion of sebum. end, depresses the epidermis. Three common problems related to sebaceous glands : Hair in human being; are continuously lost and regrown. The duration of scalp hair is 2-5 years and 1. Pimples — Sebum accumulation, causes that of the eyebrows and eyelashes is 3-5 months. growth of bacteria because it is nutritive, gets infected and results into the formation of boils and Hairs from the different parts of the body (head, pimples. chest, arm pit, beard, nose, etc.) show subtle differences. These are helpful in foren51c (crime 2. Acne — Sebaceous glands get inflamed due detection) investigations. to hormonal influence. It is one of the commonest problems in adolescents. shl Hairs also provide a sensation of touch 3. Black heads — When an open pore in the because nerve fibres extend up to their bases. skin gets clogged with dead skin cells and sebum, There are hairs along the edges of the eyelids it turns black upon oxidation by the air and is called (eyelashes) to help prevent the entry of foreign’ a black head. particles. Similarly, there are hairs in the nose, again to prevent dust particles from entering nasal 13.5.4 Sweat Glands passages. Facial hairs in human males, i.e. Each sweat gland is a simple coiled tube moustaches and beard, help in distinguishing the consisting of a deeper secretory part and an male sex (sexual dimorphism). ¥ excretory part which runs upwards to open on the surface. The outer openings are called the sweat 13.5.2 Nails pores. Their total number in the body is estimated Nails are hardened keratinous plate-like at about two million. Human races belonging to structures which grow as dead cells from the nail hotter countries usually have more sweat pofes than root, which lies below the skin at the base, those belonging to colder ones. 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Heat is lost from our body through the following The body temperature of cold-blooded four channels. (ectothermal) organisms fluctuates with the 1. Skin : About 85 per cent of the body heat is lost environmental temperature. This means that they through the skin by convection, conduction, must somehow escape both conditions of too hot radiation and through evaporation of sweat. or too cold environment and thus they hibernate 2. Lungs : Heat is lost in the warm air which is (winter sleep) or aestivate (summer sleep) as in breathed out. This loss in warm air can easily the case of frogs. be experienced by gently blowing on the back of your hand keeping the mouth wide open. ECTOTHERMAL and ENDOTHERMAL Some heat is also lost during vaporisation of The, terms cold-blooded and warm-blooded have water from the lungs. | been substituted by poikilothermal (poikilo : 3. Urine and Faeces : These substances are varying) and homeothermal (homeo : same) eliminated at body temperature. respectively. 4. Foods : Heat is also lost when cold food, water, or cold beverages are taken into the body. Human body temperature is usually about 37°C (in the mouth), it is about 1°& higher in the rectum 13.6.3 Temperature regulation and about 1°C lower in the arm pits. i The principal heat-regulating centre is located in o Our body temperature may also show variation the hypothalamus, a portion of the fore-brain. This of 0.3°C to 0.5°C in the course of 24 hours. It is part acts like a thermostat. When the body tends to lowest in the early morning and highest in the cool below the normal temperature, it switches on late afternoon. ' or speeds up the heat-producing processes, and ¢ Rise in body temperature above normal means when the body tends to get overheated, it accelerates fever and sickness and similarly, a fall in the cooling process, and switches off the heat- temperature may be dangerous. A suitable producing processes. (optimum) temperature of 35°C_ — 40°C is IN COLD WEATHER — When outside nn essential for normal body activities, especially for temperature is low; the blood vessels get narrowed the action of enzymes. The working of enzymes (vasoconstriction) (Fig. 13.4 A). This reduces the is slowed down’” with the lowering of blood supply to the skin. As a result, there is less temperature, while higher temperatures destroy loss of heat by convection, conduction and radiation, S the enzymes altogether. and also less loss of it through vaporisation of sweat. es 13.6.1 Heat production As the blood supply to the skin is reduced, the sweat glands also secrete less sweat. This is a natural There are many sources of heat production in method to conserve body heat. When our body. vasoconstriction occurs, it makes a person look pale o Chemical reactions occurring in all body cells, or bluish, because of reduced blood supply to the especially in the liver by the oxidation of glucose, skin. Simultaneously, the heat production is generally produce heat. increased by the increased metabolic rate and e Most of the heat produced in our body comes through increased muscular activity which is _ from the activity of our muscles. Vigorous sometimes in the form of shivering. activity makes you warm on a cold day and IN HOT WEATHER — When the outside overheats you on a hot day. ' temperature is high or when a person is engaged ¢ A small amount of heat comes from the in strenuous physical work, which means ingestion of hot foods and beverages. overproduction of heat within the body, the blood § Skin — “The Jackof All Trades™ ¢  ‘uone[IposeA pue uojelodens Jeems Aq ssof yeay Buowoid £Q 10 ‘UOROLISUOOOSEA PUE JB} “Jley UBNoIY) Jaujeam pjoo Ui sso Jeay Buguenaid Aq ainjesadwis} Apoq ssieinbel ung < ‘sjieu pue spue|b jeems ‘spuelb Alewwew ‘spuelb (ji0) SN0YEqas ey — UNS JO SIAIBAUSP [e108ds BAl B8 BIeyL < “1eqiosqe %00uS B SB SOAISS OS[E }| “IOJe|nsUl Jesy B SE [|oM SE BAISS8l Poo) B SE SOAISS UOIYM JoAe| 18} 8ul SI SiuLSp O} XaN